The invention is based on a brake pressure booster for vehicle brake systems. Hydraulic brake systems, having multi-circuit tandem master brake cylinders, are known (German Offenlegungsschrift No. 27 23 734); it is also known to provide such brake systems with anti-skid braking (ABS) functions, for example incorporating electrically actuatable multi-position magnetic valves, which realize the desired and necessary anti-skid control functions, in the outgoing brake pressure lines leading to the wheel brake cylinders. If there is a pressure drop, these magnetic valves close the particular pressure line from the master brake cylinder to the associated wheel brake cylinders and if needed deliver pressure fluid to the return system. If a renewed pressure rise is desired, then communication with the master brake cylinder is reestablished, along with arbitrary pressure/time courses, and with the option of keeping pressure unchanged, without either a rise or a drop as well. The prerequisite in such vehicle brake systems is always the availability of an energy source, which furnishes the brake fluid to the master brake cylinder at high pressure, so that after pressure, so that after the braking effect to be attained has been specified, this brake fluid can be fed into the pressure lines leading to the wheel brake cylinders. A separate energy source of this kind, which typically is a pump that includes an electric motor for driving the pump and appropriate pressure switches, is also required for anti-skid functions, because with the electrically actuatable multi-position magnetic valves of the prior art, brake medium or brake fluid that is under pressure is practically "lost" and must be pumped back up again later to the high pressure required for the braking process. In realizing anti-skid functions, it is also necessary to design such systems, which effect an interruption of the brake lines and an outflow of the pressure fluid in the brake lines, very reliably and in a fail-safe manner, so that if a failure occurs there will be no danger that too much high-pressure brake fluid will be drained out or that it will no longer be at all possible to furnish brake fluid to the wheel brake cylinders.
In the field of realizing anti-skid functions, it is also known (U.S. Pat. Nos. 3,690,736 and 3,371,979) to attach a parallel branch in the lines leading from the master brake cylinder to the various wheel brake cylinders, and to connect this branch with a chamber which is variable, by means of an electrically actuatable cylinder-piston assembly, in accordance with the triggering of an actuating coil counter to the pressure of a spring, so that a volume of pressure can be removed from and then returned to the brake line again; this is known as the basic plunger principle. In this case, although the brake circuit again remains closed, means are still required for interrupting the further delivery of high-pressure brake pressure fluid from the master brake cylinder during anti-skid functions, so that the removal of pressure fluid can become at all operative from the variable chamber to the wheel brake cylinders. This interruption can also be made possible by providing (U.S. Pat. No. 3,690,736) that the retreating piston simultaneously liberates a ball valve so as to close the master brake cylinder feed line. However, if the system fails at this point, then it is no longer possible to generate any braking pressure at all from the master brake cylinder, that is, by actuating the brake pedal.